Previously the same class, ActionView::TestCase::TestController, was
used to build a controller for every ActionView::TestCase class.

This caused issues when helpers/helper methods were set directly on the
controller (which from one test we seem to want to support).

This commit solves this by creating a new controller class for every
test case, which gives the controller a unique set of helpers to work
with.

Co-authored-by: John Crepezzi <seejohnrun@github.com>